Field of the Invention
The present invention relates to a high zirconia electrically fused cast refractory of high electric resistance which is excellent in heat cycle stability, less forms zircon crystals when used for glass melting furnaces, and can be used stably for a long time.
Statement of the Related Art
The electrically fused cast refractory (sometimes simply referred to also as refractory) has been used frequently as a refractory for use in glass melting furnaces.
The electrically fused cast refractory is a refractory which is dense and has excellent corrosion resistance to molten glass, and produced by melting a starting material formed by mixing main components such as alumina, silica and zirconia and minor components such as sodium compounds and boric acid in predetermined amounts in an electric furnace, casting the molten product in a refractory mold, cooling the product in an annealing material, and solidifying the same into the shape of the mold.
For example, a high zirconia electrically fused cast refractory containing 80% by weight or more of ZrO2 is used for such an electrically fused cast refractory.
Since the high zirconia electrically fused cast refractory has a high ZrO2 content and a dense texture, the refractory has high corrosion resistance to all kinds of molten glass.
Further, since the high zirconia electrically fused cast refractory has a property of not forming a reaction layer at a boundary with the molten glass, it is excellent in that defects such as stones or cords are less formed in the molten glass.
Accordingly, the high zirconia electrically fused cast refractory is particularly suitable to produce of high quality glass.
In the mineral structure of the high zirconia electrically fused cast refractory, a most portion thereof comprises monoclinic zirconia crystals in which a small amount of a glass phase fills the grain boundaries of the zirconia crystals.
Generally, the glass phase of the high zirconia electrically fused cast refractory comprises oxides such as Al2O3, SiO2, Na2O, B2O3, and P2O5.
On the other hand, zirconia crystals of the refractory transform reversibly between a monoclinic system and a tetragonal system accompanying abrupt volume change at a temperature near 1000° C. (upon temperature lowering) to 1150° C. (upon temperature rising).
A high zirconia electrically fused cast refractory with no cracks during production and upon temperature rising can be produced at a level of actual production by moderating a stress generated by the volume change accompanying the transformation of the zirconia crystals by the flow of the glass phase that fills the zirconia crystal grain boundaries.
Further, when the high zirconia electrically fused cast refractory undergoes heating or heat cycles, silica (SiO2) and zirconia (ZrO2) crystals as main components of the glass phase sometimes react to form zircon (ZrO2.SiO2) crystals.
In this case since the zircon crystals are formed in the glass phase, formation of the zircon crystals relatively decreases the glass phase. Further, as the glass phase decreases due to growing or increase of the zircon crystals, abrupt volume change of the zirconia crystals at a temperature near 1000° C. to 1150° C. becomes less absorbed.
As a result, when the zircon crystals increase exceeding a certain level, a residual volume expansion of the refractory in itself increases extremely to sometimes generate cracks due to deterioration of the strength of the refractory structure, and finally suffer from pulverization.
Accordingly, a high zirconia electrically fused cast refractory that less forms zircon crystals and is stable against heat cycles has been demanded (first subject).
Further, even a high zirconia electrically fused cast refractory which less causes zircon crystals for the refractory in itself by heating or heat cycles, tends to form zircon crystals under the condition where the refractory is in contact with molten glass. Particularly, when the high zirconia electrically fused cast refractory is used for a glass melting furnace for melting non-alkali glass such as liquid crystal display (LCD) panel glass (which may be hereinafter referred to as liquid crystal glass), zircon crystals often tend to be formed.
The zircon crystals are formed upon melting of the glass due to difference of the concentration of the constituent components between the molten glass and the glass phase of the high zirconia electrically fused cast refractory by substitution of the respective components each other.
That is, components that suppress the formation of the zircon crystals in the high zirconia electrically fused cast refractory diffuse into the molten glass. Conversely, components tending to form the zircon crystals intrude from the molten glass into the refractory. It is considered that the formation of the zircon crystals in the high zirconia electrically fused cast refractory is promoted by one or both of the reasons described above.
In a state where the zircon crystals are formed in the high zirconia electrically fused cast refractory used for the glass melting furnace to decrease the amount of the glass phase, abrupt volume change of the zirconia crystals is difficult to be absorbed at a temperature near 1000° C. to 1150° C.
As a result, when the refractory undergoes heating during operation and heat cycles due to the change of operation temperature, the residual volume expansion of the refractory in itself increases extremely, by which the strength of the structure is lowered tending to generate cracks in the refractory. The refractory is eroded selectively from the cracked portion. When erosion proceeds further, pieces of refractory intrude into the molten glass to sometimes deteriorate the quality of the glass.
On the other hand, when a high zirconia electrically fused cast refractory that less forms the zircon crystals even in contact with the molten glass is used as the furnace material, the zircon crystals are less formed and the refractory remains stable even when undergoing heating during operation of the glass melting furnace and the heat cycles due to change of the operation temperature, and cracks are less formed. Further, upon temperature lowering when the production of the glass melting furnace is interrupted, occurrence of new cracks and growing of already formed cracks can be suppressed.
Accordingly, upon re-operation after interruption of the operation, the high zirconia electrically fused cast refractory can be used again without replacing the refractory.
As described above, a high zirconia electrically fused cast refractory that less forms the zircon crystals even under the condition in contact with the molten glass has been required (second subject).
Further, for the non-alkali glass such as liquid crystal display glass (LCD), glass of a composition having higher electric resistance than usual is used for improving the property.
Accordingly, a product of high electric resistance has been required also for the high zirconia electrically fused cast refractory as the material for the glass melting furnace (third subject).
The glass phase of the high zirconia electrically fused cast refractory comprises oxides such as Al2O3, SiO2, Na2O, B2O3, and P2O5 as described above.
However, while the amount of the glass phase in the refractory is small, the property of the refractory undergoes a significant effect depending on the kind and the amount of the components that form the glass phase.
That is, it has been known that while alkali metal oxides such as Na2O in the oxides suppress the formation of the zircon crystals upon heating of the refractory in itself, cracks tend to occur during production of the refractory or upon temperature rising of the glass melting furnace constructed with the refractory, which significantly lowers the electric resistance of the refractory.
On the other hand, since oxides such as B2O3 and P2O5 provide an effect of preventing cracks during production of the refractory and cracks upon temperature rising of the glass melting furnace constructed with the refractory, they are often used together with the oxides such as Na2O. However, such oxides promote the formation of the zircon crystals contrary to Na2O. Particularly, since P2O5 has a remarkable trend, it should be used carefully.
Accordingly, a refractory comprising a glass phase having low content of alkali metal oxides (for example, less than 0.05% by weight) and relatively high content of B2O3 has relatively high electric resistance, generate less cracks during production and upon temperature rising, so that stable start-up for operation is possible.
However, zircon crystals tend to be formed upon heating the refractory in itself and, particularly, under the condition in contact with the molten glass. Accordingly, since generation of cracks and corrosion proceed during operation of the glass melting furnace, there is a subject that the stable operation for a long time is not possible and the refractory cannot be used again upon re-operation after interruption of the operation.
The present invention relates to a high zirconia electrically fused cast refractory of high electric resistance comprising a glass phase having a low content of alkali metal oxides such as Na2O and a relatively high content of B2O3.
High zirconia electrically fused cast refractories of high electric resistance and less forming the zircon crystals have been studied so far.
JP-A S63(1988)-285173 discloses a high zirconia electrically fused cast refractory of high electric resistance containing 90 to 98% of ZrO2 and 1% or less of Al2O3, not substantially containing any Li2O, Na2O, CaO, Cud, and MgO, containing 0.5 to 1.5% of B2O3 or containing 0.5 to 1.5% of B2O3, and containing 1.5% by weight or less of one or more of K2O, SrO, BaO, Rb2O, and Cs2O.
However, the refractory described in JP-A S63(1988)-285173 has high electric resistance but involves a drawback of having a large residual expansion after a heat cycle test and tending to form zircon crystals for the refractory in itself. Further, it was insufficient to suppress the formation of the zircon crystals also under the condition in contact with the molten glass. Further, the refractory does not contain CaO that controls the viscosity of the glass phase and stabilizes the glass phase and cannot moderate the stress generated during production to generate cracks in a one-side heating test to be described later.
JP-A H04(1992)-193766 proposes a high zirconia electrically fused cast refractory of high electric resistance containing 85 to 95.5% by weight of ZrO2, 3.5 to 10% by weight of SiO2, 1 to 3% by weight of Al2O3, 0 to 1.5% by weight of B2O3, 0.3 to 3% by weight of BaO, SrO, and CaO in total, 0 to 1.5% by weight of ZnO and not substantially containing Na2O and K2O (less than 0.01% by weight).
JP-A H04-193766 disclosed a refractory not substantially containing alkali metal oxides such as Na2O and K2O, containing alkaline earth metal oxides such as CaO, BaO, and SrO, having high electric resistance, showing small residual expansion even after repeating heat cycles passing through the transformation temperature of the zirconia crystals, and suppressing the formation of the zircon crystals. Further, the reference describes that ZnO has an effect of decreasing the viscosity of a matrix glass in a predetermined temperature region without increasing the rate of heat expansion of the matrix glass.
However, since the refractory has a high Al2O3 content, electric resistance was insufficient.
Further, suppression of the formation of the zircon crystals was insufficient upon heating the refractory in itself and under the condition where the refractory is in contact with the molten glass.
JP-A H09(1997)-2870 proposes a high zirconia electrically fused cast refractory with less cracks during production and cracks due to heat cycles, containing 89 to 96% by weight of zrO2, 2.5 to 8.5% by weight of SiO2, 0.2 to 1.5% by weight of Al2O3, less than 0.5% of P2O5, less than 1.2% by weight of B2O3, less than 0.3% by weight of CuO, more than 0.01 and less than 1.7% by weight of P2O5+B2O3, 0.05 to 1.0% by weight of Na2O+K2O, 0.01 to 0.5% by weight of BaO, less than 0.5% by weight of SnO2, and less than 0.3% by weight of Fe2O3+TiO2.
In JP-A H09(1997)-2870, since the content of Na2O and K2O is high, the electric resistance was insufficient.
Further, this reference describes that formation of the zircon crystals can be suppressed by adding Na2O, K2O, and BaO even when P2O5 and B2O3 that promote formation of the zircon crystals are contained. However, while BaO that suppresses the formation of the zircon crystals is contained, since P2O5 that remarkably promotes the formation of the zircon crystals is contained, suppression of the formation of the zircon crystals was not sufficient upon heating the refractory in itself. Further, Na2O and K2O that suppress the formation of the zircon crystals tend to diffuse to the molten glass under the condition where the refractory is in contact with the molten glass, suppression of the formation of the zircon crystals was not sufficient.
Further, SnO2 is not an essential component and the effect of SnO2 against cracking during production of the refractory or cracking after the heat cycle is not described at all, and the effect of adding SnO2 is not apparent.
JP-A 2008-7358 proposes a high zirconia electrically fused cast refractory of high electric resistance excellent in a heat cycle stability comprising 87 to 96% by weight of ZrO2, 0.1 to less than 0.8% by weight of Al2O3, 3 to 10% by weight or less of SiO2, less than 0.05% by weight of Na2O, 0.01 to 0.2% by weight of K2O, 0.1 to 1.0 by weight of B2O3, 0.1 to 0.5% by weight of BaO, less than 0.05% by weight of SrO, 0.01 to 0.15% by weight of CaO, 0.05 to 0.4% by weight of Y2O3, 0.1% by weight or less of MgO, 0.3% by weight or less of Fe2O3+TiO2, and less than 0.01% by weight of P2O5 and CuO.
However, even within the range described above, if the total amount of Na2O and K2O is more than 0.05% by weight, the electric resistance was insufficient. Further, while the refractory contains SrO and BaO which are the components that suppress the formation of the zircon crystals and an effect of suppressing formation of the zircon crystals is obtained upon heating the refractory in itself. However since Na2O, K2O, BaO and SrO that suppress the formation of the zircon crystals tend to diffuse into the molten glass under the condition in contact with the molten glass, suppression of the formation of the zircon crystals was not sufficient.
WO2005/068393A1 proposes a high zirconia electrically fused cast refractory of high electric resistance containing 0.8 to 2.5% by weight of Al2O3, 4.0 to 10.0% by weight of SiO2, 86 to 95% by weight of ZrO2, 0.1 to 1.2% by weight of B2O3, 0.04% by weight or less of Na2O, 0.4% by weight or less of CaO, 0.1% by weight or less of Fe2O3, and 0.25% by weight or less of TiO2.
Since the refractory has a high Al2O3 content, electric resistance was not sufficient in case of exceeding 1% by weight of Al2O3. Further, CaO content exceeding 0.2% by weight is not preferred, since this lowers the electric resistance and promotes the formation of the zircon crystals.
Further, when the content of each of the components described in the examples of this reference are investigated closely, the refractory does not contain K2O, SrO and BaO having an effect of suppressing the formation of the zircon crystals, and suppression for the zircon crystals is not sufficient upon heating the refractory in itself and under the condition in contact with the molten glass.
As described above, in the existent high zirconia electrically fused cast refractories of high electric resistance, suppression of the formation of the zircon crystals are described upon heating the refractory in itself, but formation of the zircon crystals under the condition where the refractory is in contact with the molten glass which is close to the operation condition of the glass melting furnace are not described. Therefore high zirconia electrically fused cast refractory of high electric resistance and suppression of the formation of the zircon crystals was needed.